It is desirable to quickly detect when a phase-lock loop is no longer in lock, and initiate a sweep signal applied to the controllable oscillator comprising a part of the phase-lock loop to reestablish locked conditions. A co-pending application Ser. No. 041,892 filed Apr. 23, 1987 in the name of Karl A. Ireland, and assigned to the same assignee as the present invention, shows one approach to providing this detection and sweep. The time constants of the embedded oscillator circuit used for detecting phase lock conditions had time constants wherein the transfer function of the phase-lock loop, while it was in a locked condition, peaked in excess of that allowed in a particular system using the phase-lock loop. The present invention modifies the oscillator of the referenced Ireland application, so that long time constants are seen by the system when the phase-lock loop is in a locked condition, but short time constants are switched into operation when the phase-lock loop falls out of lock, so that the oscillator may quickly proceed to oscillating at full amplitude and at a high enough frequency to make detection thereof and application of a sweep signal to the controllable oscillator occur in a reasonably short time and also to stop oscillating quickly once phase-lock is acquired.
It is, therefore, an object of the present invention to provide an adaptive oscillator circuit for use in a phase-lock loop, which oscillator alters time constants in accordance with the mode of operation of the phase-lock loop.